1. Field of the Invention
The present invention relates to an optical transceiver, in particular, the invention relates to an EMI shielding structure of the optical transceiver.
2. Related Background Arts
The U.S. Pat. No. 7,195,403, has disclosed an arrangement of the interconnection from the connector plug exposed in the external of the optical transceiver to the electronic circuit set within the transceiver. In this arrangement, the interconnection is buried within the substrate, while the top and the back surfaces of the substrate provide the ground patterns each coming in contact with the shield gasket, which is made of electrically conductive elastic material, to shield the electronic circuit in the transceiver from the external.
Another U.S. Pat. No. 7,425,135, has disclosed a mechanism to fix the flexible printed circuit board with the substrate. The flexible printed circuit board electrically connects the optical sub-assembly, such as transmitter optical sub-assembly or receiver optical sub-assembly, with the electronic circuit prepared on the substrate. Further, the multi-source agreement, titled “10 Gigabit Small Form Factor Pluggable Module Rev. 3. 1 (Apr. 2, 2003)” defines the specifications of one type of pluggable optical transceivers known as XFP transceiver.
As the transmission speed of the optical communication increases, some standard comes up to 10 Gbps and over 10 Gbps is practically designed, the electro-magnetic interference (EMI) noise leaked from the equipment becomes an important subject. As a characteristic wavelength becomes shorter, even a slight gap in the equipment, which conventionally causes no effect for the EMI leakage, results in a large EMI noise with high frequency components. The U.S. Pat. No. 7,195,403 above described has disclosed an effective mechanism to shield between the primary electronic unit within the optical transceiver and the connector plug exposed externally. However, it is inevitable for the optical transceiver to provide an optical path in a side where the optical connector is mated that opens the primary electronic unit to the outside. Thus, it is necessary for the optical transceiver capable of transmitting high-frequency signals to provide some shielding mechanism for the high frequency EMI noise in the side of the optical connector.
Moreover, in such equipment that processes the high frequency signals, a resonance frequency, which is roughly determined by the physical dimensions of the space where the electronic circuit is primarily installed therein, may partially overlap with the operational frequency of the optical transceiver. This overlapping of the resonance frequency with the operational frequency degrades the frequency characteristic of the transceiver. As the frequency spectrum of the resonance becomes sharp, the degradation in the frequency characteristic of the transceiver is apparent.
One type of the optical transceiver is used in the host system such that the transceiver is inserted into the cage prepared in the host system to mate the connector plug provided in the rear end of the transceiver with the connector installed in the deep end of the cage, which secures the communication between the transceiver and the host system. Such an optical transceiver is called as the pluggable transceiver. Because the transceiver is inserted into the cage, the outer dimensions thereof are regulated in a type of a multi-source agreement (MSA). Therefore, it is practically impossible to adjust the dimensions of the transceiver to escape from the overlapping of the resonance frequency with the operating frequency. It is necessary to shift the resonance frequency from the operating frequency, or to moderate the frequency spectrum of the resonance in the optical transceiver whose dimensions are independently determined.